Project Summary/Abstract Vectored delivery of broadly neutralizing antibodies (bnAbs) has the potential to a) achieve stringent and durable suppression in HIV individuals and b) be a successful and robust prophylactic approach. The use of recombinant adeno-associated virus vectors (AAV) for such delivery applications is ideal in many respects. AAV has an outstanding safety record in clinical trials and, as long as the delivered protein is viewed as self, it can result in continuous durable expression of the transgene product. Unfortunately, due to years of affinity maturation, bnAbs exhibit unusually high levels of somatic hypermutation and may have uncommon features that can be seen as `non-self' by the recipient's immune system. In fact, despite showing the huge promise of this approach, monkey trials from our group and others have revealed that antibody responses to the delivered bnAbs can severely limit their concentration and functionality. Building up on our previous monkey trials, what we propose here is an immunomodulatory approach aimed at avoiding the host anti-antibody responses (also known as anti-drug antibodies or ADA). Our goal is to induce immune tolerance to the delivered antibodies prior to AAV inoculation so that the desired concentrations can be consistently achieved in circulation. We plan to do this by targeting dendritic cells. Dendritic cells, as central orchestrators of the immune responses, decide the fate of the antigens they encounter. If they trigger activation of T-cells and antibody production, the antigen is set for clearance or removal. Alternatively, dendritic cells can generate anergy and tolerance. The antigen is then seen as `self' and remains. We will use an immunomodulatory therapy that targets dendritic cells in vivo, to induce antibody-specific tolerance prior to the AAV-mediated delivery of antibodies (Aim 1). We will then investigate the performance of our approach during AIDS-virus infection: a therapy trial with SHIV-infected macaques will be attempted in which a combination of bnAbs will be tolerized before being delivered with AAV (Aim 2). By eradicating or minimizing the host anti-antibody responses, we aim at making the AAV-delivery of antibodies a safe and reliable approach against HIV. If satisfactory delivery methods are found, it becomes possible to envision a) long-term control of the viral loads in the absence of antiretroviral treatment by delivering a combination of bnAbs in people and b) long-lasting protection when this approach is used in a prophylactic setting.